Hydrocracking is a petroleum refining process wherein the molecules of a hydrocarbonaceous material are partially decomposed in the presence of hydrogen to obtain a more desirable lower molecular weight hydrocarbon product. As a result of the increased demand for middle distillates such as jet and diesel fuels, commercial interest in hydrocracking has intensified because the process degrades less of the feedstock to gas and coke and thereby yields a greater proportion of middle distillate boiling range liquid products than thermal or nonhydrogen consuming cracking processes.
A crucial element of a competitive hydrocracking process is the use of a special catalyst that is highly active for both cracking and hydrogenation. A great number of these bifunctional catalysts are well known in the prior art. In general, cracking activity, i.e., a measure of the conversion of feedstock to product, is most often attributed to solid acid components present in the catalytic base. Hydrogenation activity, on the other hand, is usually credited to the various metals deposited on the catalyst base. These metals are most often selected from Groups VIB and VIII of the Periodic Table of Elements.
Popular catalysts employed in the hydrocracking industry for middle distillate production comprise a metal(s) and/or metal compounds selected from Group VIB or Group VIII of the Periodic Table deposited on an amorphous refractory inorganic oxide base material such as mixtures of silica-alumina and alumina. Recently significant commercial activity has been directed toward developing hydrocracking catalysts which comprise crystalline aluminosilicates in the catalyst base.
Generally, zeolitic aluminosilicates possess greater total acidity than amorphous oxides and therefore exhibit higher cracking activity. Because of this characteristic, zeolites are believed to impart a superior catalytic effect on hydrocarbon conversion. Research in the development of zeolitic hydrocracking catalysts has given special attention to the Y-type zeolite originally disclosed in U.S. Pat. No. 3,130,007. The catalytic mechanism of the Y-type zeolite is not fully understood. It has been observed, however, that a hydrocracking catalyst comprising zeolite Y not only exhibits increased activity for middle distillate production, but also better selectivity for the desired product middle distillate.